1. Field of the Invention
The present invention relates generally to molding machines and, more particularly, to a two-stage electric injection unit for an injection molding machine.
2. Description of the Related Art
The injection unit of an injection molding machine provides essentially two functions during the course of a normal cycle of operation; namely, injection and extruder. In a standard reciprocating screw injection molding machine, the extruder function is accomplished when the screw is rotated, gradually moving plastic melt toward the forward end of the screw, thereby creating a pressure or force to move the screw rearward to its pre-injection position as the melt accumulates. When a sufficient amount of material is accumulated ("a shot"), the screw is moved rapidly forward (without rotation) to inject the melt straight into the mold, thus performing the injection function.
The injection unit of a molding machine can also be designed as a "two-stage" system where the extruder and injection functions are performed by separate machine elements. In a two-stage injection system, the extruder or plasticizing function is still performed by a feed screw in a heated barrel, but the plastic melt is diverted into an "accumulator" (usually positioned adjacent the plasticizing barrel) rather than being conveyed from the barrel directly to the mold cavity. The accumulator is subsequently operated to perform the injection of plastic melt into the mold. The advantages of a two-stage injection unit include more uniform plastication of material, reduced wear on the screw and barrel, and the potential for higher injection pressures. The primary disadvantages are higher unit cost and material carryover from shot-to-shot that can affect part quality. More specifically, when thermoplastic material is maintained in a fluid state (above melt temperature) for an extended period, its properties will degrade to varying extents depending on the type of material, the temperature of the melt and the time it is held at the elevated temperature. The construction of the accumulator and internal piston generally determines how much material remains in the accumulator after the shot is injected into the mold.
In either type of system, the injection and extruder functions each require an associated drive apparatus in the injection unit. In hydraulic machines, movement of the screw for the injection function is typically performed by one or more hydraulic cylinders, while the rotation of the feed screw for extruder run is normally accomplished by a hydraulic motor. More recently, electric motors combined with mechanical systems have been used as the direct power source for reciprocating screw injection units. Notably, these prior art electric systems have used separate motors for each function; i.e., one motor for rotating the feed screw and a second motor in combination with a mechanism, such as a ball screw, to convert rotary motion into the linear movement required to move the screw forward for injection.
Accordingly, as is typical when new technology is applied to existing products, the effort has been to maximize the execution of the previous injection system technology so as to limit risk and retain product identity. This is especially true in all-electric injection molding machine design where hydraulic motion control has been replaced with electromechanical motion control. As a result of this limited design approach, many important advantages of electric variable speed motor drives have not been realized in their application to injection molding.
For example, it is generally known that the hydraulically driven reciprocating screw injection unit design has a shot size consistency and repeatability capability of approximately .+-.0.2%, due to hydraulic system fluctuations mentioned above and inconsistency of the non-return valve at the end of the screw (the non-return valve is a necessary component to the proper functioning of the reciprocating screw design). Given that all of the all-electric machines in the market today have a reciprocating screw, the potential for reducing shot size variation has been limited to the improvement of positioning repeatability of screw forward axis alone.
It is well established that simply replacing hydraulic drive trains with electromechanical drive trains provides significant, measurable improvement in repeatability, stability, and accuracy of the driven device. This is a result of reducing the number of components in the drive train, elimination of inherent variations in the hydraulic fluid as a function of temperature, viscosity changes due to ultimate chemical breakdown of the oil itself, eventual increasing concentration of contaminants, and so forth. However, while simply replacing the hydraulic drive train components with servo-electrical/mechanical components provides desirable performance improvement, the full potential improvement has yet to be realized.
The potential for improvement is particularly evident in reciprocating screw injection units having relatively large shot capacity. While the increased shot size is relatively simple in reciprocating screw hydraulic machines, the substitution of electric motors and ball screws for hydraulic cylinders ultimately becomes impractical due to the excessive cost of the large ball screws required (to reciprocate the screw for injection). Although the size of the ball screws can be reduced by using two screws in tandem, the costs for the screws and associated components remain excessively high. In addition, the construction of electric reciprocating screw injection units that have capacities to match the range of hydraulic units available would require ball screws of sizes that are untested and, in fact, exceed current manufacturing capabilities.
In addition, the processing requirements for injection molding commercially significant plastics materials involve injection pressures of at least 15,000 psi, and frequently up to 30,000 psi. Given that availability and cost of ball screws are more affected by diameter rather than length, ball screws in excess of six inches in diameter are virtually unavailable in commercial quantities--which has severely limited the advance of all-electric designs beyond about 32 ounces shot capacity. For example, a typical 100 ounce shot capacity hydraulic injection unit would have a reciprocating screw of about 4 inches in diameter to generate 20,000 psi. An all-electric (reciprocating screw) equivalent would need a ball screw far in excess of six inches in diameter to carry the load. In fact, the largest commercially produced all-electric injection unit in the world today uses two ball screws 6.5 inches in diameter to support the load requirements of a 3.5 inch diameter reciprocating screw operating to inject up to 77 ounces of melt at a maximum injection pressure of 22,000 psi.
Ball screw performance and durability suffer in reciprocating screw injection applications. To get optimum useful life from the ball screw, minimum levels of ball circulation and lubrication circulation must be accomplished. However, the reciprocating screw design is limited to relatively short injection stroke, because longer strokes induce unacceptable plastic processing variations that result from the decreasing effective screw length to diameter ratio (L/D) as the screw retracts while building the shot volume for injection. By current standards, rarely does the injection stroke exceed five times the screw diameter in a reciprocating screw design. Furthermore, prior art (hydraulic) two-stage injection units have adhered to roughly the same ratio for the stroke and diameter of the accumulator piston.
Typically, the size of the "shot" processed in most reciprocating screw injection units would probably be about 25% of the maximum. (This results from the fact that the screw is sized by plasticizing requirements rather than shot capacity.) Using the 25% limitation for purposes of illustration, in a reciprocating screw, all-electric injection unit, maximum ball screw travel would likely be limited to one screw diameter or less for a majority of the machine's service life. Ball screw leads are typically one-fourth to one-half the diameter of the ball screw and are usually designed to have at least three complete thread revolutions under load. For example, if an injection unit is traversing one screw diameter for injection, and the injection axis ball screw is twice the diameter of the injection screw, the loaded balls in the mechanism never fully circulate to unloaded positions and some of the unloaded ball do not move to a loaded position. This results in uneven wear of the components and the natural lubrication that would occur from complete circulation of the balls must be supplanted with frequent, external lubrication. Accordingly, ball screw life in a reciprocating screw injection unit is less than it would be in an application where there is full circulation of the balls.
It should be noted that the relatively large diameter and short stroke of the reciprocating screw injection unit facilitates high speed injection; however, a high torque motor is required to produce the desired injection pressures. Since horsepower is a function of the product of motor torque and RPM, the high torque requirement means that high horsepower motors are required to drive the injection mechanism,
Another consideration is that the floor space occupied by an injection molding machine has become an increasingly important criteria. As the resources once available for facilities are diverted to other assets to increase productivity, the length, width and height of a machine has become increasingly important consideration among competing machine designs. In all-electric machines, the injection ball screw is most advantageously arranged in line behind the injection piston. In the case of the reciprocating screw, the plasticizing screw is the injection piston, and already has a length that is fifteen to thirty times its diameter because of plasticizing requirements. Since it generally desirable to lengthen injection stroke as much as possible, positioning a ball screw in-line with the plasticizing screw for the injection stroke results in a machine of undesirable overall length.
Besides the need for increased capacity in electric injection units, there is potential for improvement in durability, repeatability, stability, and accuracy of the driven device, as well as a reduction in overall length of the machine, if a way can be found to overcome the obstacles presented by limiting application of electro-mechanical technology to reciprocating screw injection units. Furthermore, in order to take full advantage of the desired improvements, a new construction for an electric injection unit should not introduce the material carryover problems typically associated with prior art two stage injection units.